1. Field of the Invention
This invention relates to hot water heating systems, and more particularly concerns water heating systems in households where hot water is used intermittently and at times is not used at all for relatively long periods.
2. Description of the Prior Art
In domestic hot water systems now in use, water with a temperature in the range of about 34.degree.-70.degree. F. is drawn from a source such as a city main, and stored in a tank that acts as both a heater and a storage unit. A control unit, which is under the direction of thermostats, causes the feeding of fuel to a burner that is associated with the heater to bring the temperature of the water in the tank up to a desired temperature in the range of 140.degree.-180.degree. F. Once the desired temperature has been obtained, the thermostats will cycle on and off to activate or de-activate the burner to maintain the water in the desired temperature range.
An obvious disadvantage of this arrangement is that a considerable amount of fuel must be used to initially raise the temperature of the incoming water. Also, since heat is lost through the insulated wall of the tank, the burner must be repeatedly activated to maintain the temperature within the selected range even when no water is drawn out of the tank by a member of the household.
A solar system has been proposed for lessening or eliminating the fuel expenditure incurred in initially warming up the relatively cold water from the city mains. Such a solar system involves the steps of directing the relatively cold water from the city main into a first storage tank, withdrawing the water, circulating it through solar heating panels and redirecting it into the tank, thus providing a tank of preheated water at a minimum temperature of about 100.degree. F. The tempered water is then withdrawn from the storage tank and directed into a second tank where a conventional control and burner maintain the water at the desired temperature, for example, 160.degree. F. While this system results in a savings in fuel in the initial heating of the relatively cold water, it also has the disadvantage that, during long periods of non-use, fuel is wasted due to the necessity of the burner cycling on and off to maintain the water in the second storage tank at the desired temperature.
To solve the problem of heat-loss from large quantities of stored water, the practice of maintaining large reserves of fuel-heated water has been eliminated in some European and Latin American countries. These places use a quick-heat device that turns on a burner in a heat-exchanger while the water is being used. The burner is turned on by a water-flow transducer so that fuel is expended only when water is being drawn by the user. While this arrangement eliminates the fuel loss due to the constant cycling needed to keep a quantity of stored water at a desired temperature, it too is wasteful of fuel since the heat exchangers that are used require that the burner comes on at full output any time the unit is being used regardless of the amount of water being drawn. Further, if a small amount of water is being used, as when a person is washing his hands, the unit will not come on at all. Accordingly, in order to energize the unit and obtain hot water, the user must open the faucet wide so that the heater will turn on. This procedure is wasteful of water and fuel. Also, if two users want hot water at the same time, the water often flows through the heat-exchanger too fast for the water to be raised to the desired temperature. The use of a solar heat preheater would not eliminate the excessive use of fuel in such a quick-heat system, since the heat exchanger turns on at full power regardless of the temperature of the water coming into the heat exchanger.